The present invention relates to an electrophotographic image forming apparatus such as a copying machine or a printer.
FIG. 12 shows a typical conventional image forming apparatus.
A photosensitive drum 101 is rotatively driven. After being uniformly charged to the negative polarity by a primary charging device 102, the peripheral surface of the photosensitive drum 101 is exposed to a laser beam 103. As a result, an electrostatic latent image which reflects image data is formed. The electrostatic latent image is developed in reverse into a toner image by a developing device. More specifically, developing devices 104a, 104b, 104c and 104d, which contain negatively chargeable yellow, magenta, cyan and black toners, respectively, are mounted in a rotary 104, which is rotatable about its axis to position one of the developing devices, that is, the developing device for developing the electrostatic latent image currently present on the peripheral surface of the photosensitive drum 101, at the latent image developing zone where the peripheral surface of the developing device squarely faces the peripheral surface of the photosensitive drum 101. For example, in order to develop the electrostatic latent image correspondent to the yellow component of the image to be formed, the rotary 104 is rotated to position the yellow color developing device 104 at the latent image developing point so that yellow toner is adhered to the latent image, that is, to develop the latent image into a yellow toner image.
The thus formed yellow toner image is transferred (primary transfer), in a primary transfer station 106a, onto an intermediary transfer belt 105 by applying primary transfer bias to a primary transfer roller 109. The toner which remains on the peripheral surface of the photosensitive drum 101 after the primary transfer process is removed by a cleaning apparatus 107.
The aforementioned charging process, exposing process, developing process, primary transfer process, and cleaning process are carried out for the rest of the color components, that is, magenta, cyan, and black color components. As a result, four toner images of different color are overlaid on the intermediary transfer belt 105.
Then, the four color toner images are transferred (secondary transfer) all at once in a secondary transfer station 106b by a secondary transfer roller 110, onto a transfer medium P, which is conveyed from a sheet feeding station (unillustrated).
After the secondary transfer process, the transfer medium P is conveyed to a fixing apparatus (unillustrated), in which the four color toner images are fixed to the surface of the transfer medium P by heat and pressure. Then, the transfer medium P is discharged into a delivery tray (unillustrated).
The toner which remains on the intermediary transfer belt 105 after the secondary transfer process is removed by a cleaner 108.
Some of the image forming apparatuses are provided with a mechanism which automatically controls the magnitude of the development bias applied to the developing sleeves of the developing devices 104a, 104b, 104c and 104d, in order to adjust image density so that image quality is improved. In such an image forming apparatus, charge bias applied to the primary charging device 102 is also varied in magnitude in accordance with the magnitude of the development bias.
However, as the primary charge bias is varied as described above, toner is scattered, detrimentally affecting the final image in terms of color accuracy; degrading the image quality, in particular, in the areas of the image in which the toner images of different color are literally overlaid. This is thought to occur due to the following reason. That is, if the difference between the electrical potential level to which the photosensitive drum 101 has been charged and the voltage level of the primary transfer bias becomes excessive, it becomes impossible for a proper image transfer electric field to be formed; electrical discharge occurs in the non-image portion, detrimentally affecting the image transfer process. On the other hand, if the aforementioned difference is excessively small, not only does a proper transfer electric field fail to be formed, but also it becomes impossible to give electrical charge even to the non-image portion of the intermediary transfer belt 105 during the processes in which the toner images of different color are overlaid on the intermediary transfer belt 105, and therefore, it becomes impossible to form a barrier composed of electrical potential, to prevent toner from scattering. As a result, images are inaccurately formed in terms of color.
The object of the present invention is to provide an image forming apparatus capable of preventing toner particles from scattering from the toner images after the toner images are transferred from an image bearing member onto an intermediary transfer member.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.